A mechanical vibration switch is a device that senses mechanical vibrations on various types of machinery and changes state when a threshold vibration level is reached. The purpose of the switch is to either provide an alert that the machine is vibrating unacceptably or to shut the machine down so that damage does not occur. Referring to FIG. 1, a prior art mechanical vibration switch 10 typically includes a small rare earth magnet 13, a magnetic material part 16 (usually a steel plate), an inertial mass 19, a spring 22, and an electrical relay 25. The magnetic material part 16 is mounted to the main switch mechanism 28, and its position relative to the magnet 13, in the set position, is adjustable by means of a screw or the like (not shown). The magnet 13 is mounted on a bar/lever 31 that is acted on by the spring 22, and the lever arm 31 is also mechanically connected to the throw of the electrical relay 25. The bar 31 may rotate about a pivot point 32 in the direction of arrow 33. In the set position, the electrical relays 25 are in one state, either NO (normally open) or NC (normally closed), and the relays 25 change state depending on the position of the bar 31. The bar 31 is also resting against a mechanical stop 34 in the set position. The mechanical stop 34 is also part of a sprung inertial mass mechanism. When the mechanical switch is in the set mode, the position of the magnetic material part 16 is adjusted so its distance d (gap) from the magnet 13 is such that the mechanical vibration switch 10 remains in the set position, but the magnetic part 16 is spaced a sufficient distance away from the magnet 13 so that the switch will change states when a threshold vibration level is encountered.
The sprung mass 19 (M) exerts an inertial force (F) on the bar 31. If the inertial force (F) plus the spring force Fspring become greater than the magnetic force Fmagnet holding the switch in the set position, then the switch will change states. Thus, as vibration increases, the inertial force (F) increases until sufficient vibration is encountered to trip the switch. When the switch trips, the bar 31 moves the electrical relay 25 (relay throw) to the opposite position which changes the state of the contacts (relay) thus warning of the machine problem or shutting the machine down.
The common surface area S of the surface on the magnetic material part 16 facing the magnet 13 remains constant and the distance d is adjusted in the direction of arrows 39 to adjust the sensititivity of the switch 10. The major problem with prior art mechanical vibration switch designs is that the adjustment of the force required to change the state of the switch is highly nonlinear with the distance d between the magnet 13 and the magnetic material part 16. This non-linear relation is illustrated by FIGS. 2 and 3. FIG. 2 shows a plot of distance d versus Fmagnet. This graph shows that the force of the magnet drops in a non-linear manner as the distance d increases. Because of this non-linear relationship, the sensitivity of traditional mechanical switches is frequently set too low to be effective in protecting rotating machinery, and particularly when the machines operate at slow speeds (i.e., <6000 RPM).